RFR (S) 8241351: Shenandoah: fragmentation metrics overhaul

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8241351

Current fragmentation computations have flaws:
 - fragmentation metrics are there to drive mutator behavior, but it captures evac-reserve;
 - external fragmentation do not count humongous objects spanning a single region
 - external fragmentation takes "total space" over all regions, while using "free space" only for
completely empty ones

These flaws make gc/oom/TestThreadFailure.java fail with JDK-8241139 changes.

Fix:
  https://cr.openjdk.java.net/~shade/8241351/webrev.01/

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey